Conventional, constant diameter resonant tube, thermoacoustic cooling devices have not been successfully applied to cryogenic temperatures. This is because piezoelectric drivers are used and they become less efficient at lower temperatures. For several reasons the magnitude of the piezoelectric effect (piezo gain) is dependent on the temperature. The piezoelectric effect is very stable at approximately room temperature. However, at cryogenic temperatures it reaches approximately 20% to 30% of its room temperature value.
Conventional, constant diameter resonant tube, thermoacoustic cooling devices suffer from several inefficiencies. First, the hysteresis of piezoelectric (PZT) drivers makes them less efficient than electrostatic drivers. Second, a constant diameter resonant tube (resonator) suffers from harmonic induced inefficiencies. Third, the assembly of the PZT driver, resonator and associated Micro-electromechanical Systems (MEMS) stack, can be difficult to directly integrate with electronics through wafer level bonding. The integration is difficult because these components may have to be assembled at component-level, instead of wafer-level, which is very costly and does not realize the benefits of batch-fabrication of MEMS technology.